From the viewpoint of protection of the global environment, many of boiler-based heating apparatuses that burn fossil fuel for heating have recently been substituted with operation heat pump-based air-conditioning apparatuses that utilize air as heat source, even in cold districts.
The heat pump-based air-conditioning apparatus provides heat not only from an electrically driven compressor but also from air, and therefore the heating operation can be performed with added efficiency.
Nevertheless, when outdoor temperature drops frost is formed on an outdoor heat exchanger acting as evaporator, and hence defrosting has to be performed to melt the frost formed on the outdoor heat exchanger.
To defrost the refrigeration cycle may be reversed, however in this case the heating operation for an indoor space is suspended during the defrosting, which degrades comfort in the indoor space.
Accordingly, as one of methods for performing heating operation during the defrosting, there has been proposed a technique of dividing the outdoor heat exchanger so as to utilize a part thereof for the defrosting, while utilizing the other part as evaporator to remove heat from air thereby performing the heating operation (see, for example, Patent Literature 1, Patent Literature 2, and Patent Literature 3).
With the technique according to Patent Literature 1, the outdoor heat exchanger is divided into two heat exchanger sections and, when one of the heat exchangers is to be defrosted, an electronic expansion valve located upstream of the heat exchanger section to be defrosted is closed. Then a solenoid on/off valve of a bypass pipe that conducts refrigerant from the discharge pipe of the compressor to the heat exchanger section is opened, so as to allow a part of high-temperature refrigerant discharged from the compressor to directly flow into the heat exchanger section to be defrosted. When the defrosting of one of the heat exchanger sections is finished, the other heat exchanger section is defrosted.
In this process, in the heat exchanger section being defrosted, the defrosting is performed with the refrigerant pressure set to the same pressure as the suction pressure of the compressor (low-pressure defrosting).
With the technique according to Patent Literature 2, a plurality of heat source units and at least one indoor unit are provided, and setting of a four-way valve is reversed with respect to the direction in a heating operation, only in the heat source unit that includes the heat source-side heat exchanger to be defrosted, so as to allow the refrigerant discharged from the compressor to directly flow into the heat source-side heat exchanger.
In this process, in the heat source-side heat exchanger being defrosted, the defrosting is performed with the refrigerant pressure set to the same pressure as the discharge pressure of the compressor (high-pressure defrosting).
Further, Patent Literature 3 discloses a high-pressure defrosting technique that employs a defrosting apparatus that can also serve as evaporator, installed windward of the outdoor heat exchanger, and an electronic valve that inhibits the refrigerant flowing out of the defrosting apparatus in the defrosting process from reversely flowing to the indoor unit, provided between the defrosting apparatus and the compressor discharge pipe.
In addition, with the technique according to Patent Literature 4, the outdoor heat exchanger is divided into a plurality of parallel heat exchangers, and a part of the high-temperature refrigerant discharged from the compressor is alternately introduced into each of the parallel heat exchangers to thereby alternately defrost the parallel heat exchangers. This technique enables the heating operation to be continuously performed without reversing the refrigeration cycle. The refrigerant supplied to the parallel heat exchanger subject to defrosting is injected from an injection port of the compressor.
In this process, in the parallel heat exchanger being defrosted, the defrosting is performed with the refrigerant pressure set to a pressure lower than the discharge pressure of the compressor and higher than the suction pressure thereof, more specifically a pressure corresponding to a saturation temperature slightly higher than 0 degrees Celsius converted from the pressure (medium-pressure defrosting).